Method, entity and program for transmitting communication signal frames

ABSTRACT

The invention relates to a method implemented by a communicating entity in a packet-switched network, comprising at least one port for transmitting communication signal frames comprising a first type of frames, intended to be transmitted in a plurality of streams for which a traffic shaping is defined, and a second type of frames, for which no traffic shaping is defined, each frame being able to be fragmented so as to transmit a fragment only of a frame of said second type. The communicating entity stores a plurality of first queues of frames of the first type, the first queues being associated respectively to said plurality of streams, and at least one second queue for frames of the second type. The entity further schedules transmissions of first type frames, and between at least two first type frames, transmission of at least a fragment of at least one second type frame.

TECHNICAL FIELD

The invention relates to packet-switched networks management.

BACKGROUND ART

Packet-switched networks are increasingly used for industrial controlapplication thanks to the introduction of Layer-2 features that allowthe transport of control data that cannot suffer latency and transferdelay variation.

For instance, low-latency sampling data, (closed loop) control and imagestreaming (e.g. image processing) have very stringent latencyrequirements. Image streaming and associated processing as a part of acontrol loop has greater requirements than best effort transport couldprovide in a converged network. In parallel, best effort stream is nottime-critical, but provides a constant source for interference stream.

Solutions have been progressively developed, in particular for theadaptation of switched Ethernet to the requirements of industrial fieldbusses: EtherCAT, Ethernet Powerlink, TCnet, PROFINET, etc.

All these solutions rely on specific additions to the standard Ethernetprotocol that provide support for scheduled streams.

The common scheme used by these adaptations is based on an organizationof the transmission multiplex in time windows, each window beingreserved for a specific stream. The number and repetition frequencies ofthe time windows are determined according to the requirements of theapplications.

Considering that the (industrial control) applications generatingscheduled stream have a periodic activity, the transmission multiplex isorganized in periodic cycles. Referring to FIG. 1, each cycle contains aseries of time windows STW reserved for scheduled (low-latency) framesSF transmission, the rest of the transmission opportunities beingdevoted to non scheduled streams.

SUMMARY OF INVENTION Technical Problem

However, some other control applications have less stringent latency orjitter requirements: embedded automotive control networks for exampleare among these applications. In these environments, the extra networkengineering required by the computation of the schedules and thesynchronization of traffic sources with the network can be avoidedprovided the chosen traffic management method guarantees limited jitter,bounded latency and zero-packet-loss to the critical control streams.

Traffic shaping according to a predefined data rate reserved throughoutthe network and negotiated by the source with the network is a techniquethat is well adapted to achieve these performance goals. It providesmeans to guaranty transmission delay bounds and fairness between theshaped streams.

To further reduce potential sources of latency due to the interferencebetween shaped and non-shaped streams, pre-emption mechanisms can alsobe introduced. Preemption intervenes at the transition between anon-shaped stream transmission opportunity and a shaped streamtransmission opportunity, as shown in FIG. 2. In particular, upon such atransition, the beginning of a shaped stream transmission opportunitySFTO can overlap the end of the transmission opportunity of an unshapedstream. In such a case, the frame of the shaped stream SF cannot betransmitted until the current transmission of a frame UF of a non-shapedstream ends.

As still shown on FIG. 2, by pre-empting the transmission opportunity ofthe residual part of the unshaped frame UF, it is possible to avoidadditional delay to be suffered by the shaped stream SF. To facilitatethe pre-emption operation, frames of the unshaped stream can betherefore fragmented (as shown in the lower part of FIG. 2, with thetransmission of frag#1) causing the transmission of the remainingfragment(s) (frag#2 in the example of FIG. 2) to be delayed until thetransmission opportunities of the shaped streams are complete.

So far, in order to provide efficient interworking between scheduled andnon-scheduled streams, it has been proposed:

A first type of standard as IEEE802.1Qbv, defining a scheduling schemerelying on the reservation of time windows for different types ofstreams (scheduled and non-scheduled);

A second type of standard as IEEE802.3br and IEEE802.1Qbu defining apre-emption scheme that specifies the fragmentation mechanisms appliedto so-called “normal” frames upon the concurrent transmission ofso-called “express” frames; typically belonging to scheduled streams.

IEEE802.1Qbv is based on a periodic calendar table, of which each entrydefines a time window reserved for the transmission of a particularclass of stream (e.g. scheduled or non-scheduled).

These standards provide then a complete framework for the transport ofperiodic scheduled streams with minimal latency, a similar service tothose provided by the industrial standards cited above.

The mechanism specified in IEEE802.3br allows a MAC Client that has an“Express” traffic (here shaped streams) to preempt “Normal” frames'transmission (belonging to unshaped streams), before the transmissionopportunities of shaped traffic starts. When a frame belonging to ashaped stream is scheduled for transmission, it can be transmittedimmediately.

In addition, the IEEE802.3br standard defines a per-hop fragmentationand reassembly scheme of Normal frames that allows stopping thetransmission of a Normal frame to give a transmission opportunity to anExpress frame. Normal-frame fragments and Express-frames aredistinguished by their preamble length and a “Start of Frame Delimiter”field. This allows non-express frames (Normal-frames) not to wait for atoo long timeslot before being transmitted. This results in a limitedadded latency for Normal flows and a better use of the link capacity.

It is to be noted that only a single Normal-frame can be fragmented atany time (on the same link or in the same transmit port).

The minimum preempted fragment size is 64 bytes. Therefore, a packetwith a length less than 128 bytes cannot be preempted, nor fragmentedfinally (without letting finally a fragment of less than 64 bytes).Moreover, non-final fragments have a length multiple of 8 bytes.

So far, the addition of residual latency due to interfering frames orfragments belonging to non scheduled stream can be avoided thanks to theavailability of the exact times of beginning and end of transmissionwindows dedicated to normal (non-express) streams. Indeed, knowing whenthe next express traffic transmission window starts makes it possible toselect the normal frame or fragment to be transmitted without anyoverlap with the express traffic transmission window.

However, when critical data transfers are supported by shaped or atleast rate-controlled streams (even if such transfers do not requireultra-low latency), no mechanism is defined to guaranty that thesestreams do not suffer from additional latency due to interfering withnon-critical frames or fragments transmission.

Using the state-of-the-art solution based on the current fragmentationstandard (IEEE 802.3br and 802.1Qbu), multiplexing of shaped andunshaped streams can take benefit of the pre-emption but will stillsuffer from residual latency due to the transmission of frame fragmentsbelonging to unshaped streams.

Solution to Problem

The invention aims to improve the situation.

To that end, the invention proposes a method implemented by computermeans of a communicating entity in a packet-switched network, saidcommunicating entity comprising at least one port for transmittingcommunication signal frames, said frames comprising:

-   -   a first type of frames, intended to be transmitted in a        plurality of streams for which a traffic shaping is defined, and    -   a second type of frames, for which no traffic shaping is        defined, each frame being able to be fragmented so as to        transmit a fragment only of a frame of said second type.

More particularly, the communicating entity stores a plurality of firstqueues of frames of the first type, said first queues being associatedrespectively to said plurality of streams, and at least one second queuefor frames of the second type. The communicating entity can thereforeschedule transmissions of first type frames, and between at least twofirst type frames, transmission of at least a fragment of at least onesecond type frame. Moreover, to that end, for deciding to transmit afragment only of a second type frame, the communicating entity canfurther determine whether the size of the fragment to be transmitted isgreater than a threshold, and also whether the size of a remainingfragment of said second type frame is greater than that threshold.Otherwise, if these two conditions are not met, the communicating entityprevents from fragment transmission.

Therefore, thanks to the implementation of the invention, it is possibleto organize the transmission of frames belonging to shaped streams (theaforesaid “first type” frames) so that the beginning and end time oftransmission opportunities reserved for shaped traffic are dynamicallydetermined along the transmission of the shaped streams and provide thetime boundaries for the transmission opportunities of unshaped fragmentsor frames (the aforesaid “second type” frames).

In a possible embodiment, each of said first and second queues is storedin a first-in-first-out type buffer and a queue to which a first typeframe is assigned is determined according to the stream to which thatfirst type frame belongs, depending on features of said first typeframe. Typically, information given in a header of the frame can be usedto determine the type of that frame (belonging to shaped traffic or tounshaped traffic). Furthermore, features of said first type frame caninclude its length and a current rate of the stream to which said firsttype frame belongs. Therefore, the time taken for the transmission ofeach first type frame in its queue can be calculated (classically bydividing the length of frame by the stream rate), and respectivetransmission times can be scheduled accordingly for the first typeframes.

Consequently, in this embodiment, the frame belonging to a shaped streamcan be selected for transmission by comparing scheduled transmissiontimes (noted TTTi hereafter) of frames placed at the head of each firstqueue, the frame associated to the smallest scheduled transmission time(noted miniTTTi) being selected as a next candidate for transmission.

In an embodiment where the communicating entity can perform a timecounting for determining a current time, the communicating entity cantherefore implement transmission of said next candidate if:

-   -   the scheduled transmission time associated to said next        candidate is smaller or equal to the current time, and if    -   no other first type frame is currently being transmitted, and if    -   no second type frame or fragment is currently being transmitted.

The communicating entity can determine furthermore whether a second typeframe or frame fragment is currently being transmitted, and if yeswhether that second type frame or frame fragment can be furtherfragmented for interrupting a transmission of said second type frame orframe fragment, and allowing thereby a transmission of the aforesaidnext candidate.

In this embodiment, the communicating entity can implement afragmentation of that second type frame or frame fragment:

-   -   if the size of said second type frame or frame fragment which        transmission is to be interrupted is greater than the aforesaid        threshold, and    -   if the size of a frame fragment to remain in said second queue        after the interruption is greater than that threshold.

In an embodiment, for the purpose of the second type frames'transmission, the communicating entity can perform a time counting fordetermining a current time and implementing a second type framefragmentation in view of fragment transmission:

-   -   if no first type frame is currently being transmitted, and    -   if no first type frame placed at a first queue head has a        scheduled transmission time smaller or equal to the current        time.

Moreover, a provision can be made so as to limit latency of the firsttype frames' transmission and to that end, the communicating entity caninsert a probe frame at the head of at least one of the first queues,preferably the one comprising the next candidate for transmission, so asto avoid any latency of transmission of that next candidate.

The present invention aims also at a communicating entity, comprising atleast one port for transmitting communication signal frames in apacket-switched network, and further comprising a computer circuit forperforming the method as defined above. An example of embodiment of sucha communicating entity is shown in FIG. 4 commented below.

The present invention aims also at a computer program product,comprising instructions for performing the method as defined above, whenrun by a processor. A general algorithm of such a computer program canbe illustrated by way of an example by the flowcharts corresponding toFIGS. 5, 6, 9, 10 commented below.

More generally, the present invention is illustrated by way of example,and not by way of limitation, in the figures of the accompanyingdrawings, in which like reference numerals refer to similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cyclic organization of a transmission multiplex accordingto prior art.

FIG. 2 shows unshaped frame fragmentation guarantying shaped frametransmission.

FIG. 3 shows shaped and unshaped queues in a transmission port (or“output port” hereafter) of a communicating entity according to thepresent invention.

FIG. 4 shows schematically a computer circuit (FPGA, integrated chip, orany other computer circuit) of a communicating entity according to anembodiment of the present invention.

FIG. 5 shows operations performed by an input port and/or any other partof the communicating entity generating frames to be transmitted by thetransmission port of the communicating entity.

FIG. 6A shows operations performed by the communicating entity fortransmitting a shaped frame from its transmission port.

FIG. 6B shows operations performed by the communicating entity fortransmitting a shaped frame from its transmission port.

FIG. 7A shows transmission conditions for a shaped frame, in consistencewith the embodiment of FIGS. 6A and 6B, in case of fragment transmissionof unshaped frame, the scheduled transmissions time being here fromright ([miniTTTi]1) to left ([miniTTTi]2).

FIG. 7B shows transmission conditions for a shaped frame, in consistencewith the embodiment of FIGS. 6A and 6B, in case of impossibility totransmit a fragment of unshaped frame, the scheduled transmissions timebeing here from right ([miniTTTi] 1) to left ([miniTTTi]2).

FIG. 8 shows transmission and fragmentation conditions for an unshapedframe.

FIG. 9 shows operations performed by the communicating entity fortransmitting an unshaped frame from its transmission port, inconsistency with FIG. 8, in the case where the transmission of anunshaped frame or fragment is already in progress (arrow Y from testS26).

FIG. 10 shows operations performed by the communicating entity fortransmitting an unshaped frame from its transmission port, inconsistency with FIG. 8, in the case where no transmission of anunshaped frame or fragment is currently in progress (arrow N from testS26 starting FIG. 10).

DESCRIPTION OF EMBODIMENTS

The implementation of the invention makes it possible to limit thelatency experienced by the shaped streams in the successive multiplexingstages encountered in the network. For that purpose, it takes advantageof a time-based scheduling mechanism used to dynamically define thetransmission opportunities dedicated to the shaped streams inconjunction with pre-emption applied to the unshaped streams. The timinginformation provided by the time-based scheduling mechanism enablesdetermining the size of frames or fragments candidate for transmissionbetween shaped stream transmission opportunities.

Hereafter, express frames as defined in standard IEEE802.3br are framesbelonging to shaped streams.

Normal frames as defined in standard IEEE 802.3br are frames belongingto unshaped streams (that typically support a so-called “Best Effort”transport service).

Normal frames can be segmented in fragments as per the standardIEEE802.3br.

The multiplexing scheme and involved parameters are described below withreference to FIG. 3.

Each shaped stream i is associated with a transmission rate Ri, and aqueue Qi in an egress port of each communicating entity of thepacket-switched network. In an embodiment, a communicating entity can bea multiplexing node through which the stream passes. In an alternativeembodiment, a communicating entity can be a terminal transmittingreceived frames in a peer-to-peer communication configuration, and/orsimply emitting its own frames.

The aforesaid transmission rate Ri is expressed in bit per second. Forall shaped streams i, the sum of Ri is less than the total link (orport) transmission rate ρ. Furthermore, a packet to transmit is supposedto be made of octets, each octet corresponding to 8 bits, and thisexplains the value “8” which appears later in the drawings (for exampletest S35 of FIG. 10 commented below).

Ri is reserved in the successive nodes traversed by stream i by means ofsignalling or network administration.

Packets of stream i are stored in memory buffers organized as FIFO(First In First Out), defining an order in queue Qi.

Time is represented by a variable T that is incremented every bittransmission time (at the link rate) and is usually the same for alllinks and ports of a node.

Each shaped stream i is associated with a stream context SCi thatcontains the variables required for the shaping mechanism, e.g.:

The status of the associated shaped transmit queue in the output port,Stat(Qi), which can take the values empty (0) or not empty (1);

A Theoretical Transmission Time of the frame stored at the head of theassociated shaped transmit queue, noted hereafter TTTi

For the sake of simplification here, all unshaped streams are allocatedwith a single dedicated queue: the unshaped transmit queue Qu thatstores the frames in FIFO order.

A queue context is associated with the unshaped transmit queue, UC,containing three variables:

The status of the unshaped queue, Stat(Qu), which can take the valuesempty (0) or not empty (1);

The size of the frame or remaining fragment stored at the head of theunshaped queue: sizeofQuHead;

The number of bytes transmitted since the beginning of the currentunshaped frame or fragment transmission: sizeofCurrFrag, updated uponthe transmission of each byte.

Considering the transmission of frames belonging to unshaped streams, afragment cannot be shorter than a defined minimum fragment sizeMinFragSize. Typically, a frame fragment shall include at least:

-   -   a header,    -   an indication of a position of the fragment (at least first        fragment, or intermediate fragment, or last fragment of a        frame),    -   a CRC code at the end of the fragment, and hopefully data in        between.

In fact, the minimum size MinFragSize cannot be less than a sizecorresponding to these requirements, but more generally cannot be lessthan the minimum size of an Ethernet frame (e.g. 64 bytes as previouslyindicated).

The transmission of a shaped frame is indicated by the ShapedTxBusyflag, set to 1 during the transmission, and reset to 0 otherwise.

The transmission of an unshaped frame or fragment is indicated by theUnshapedTxBusy flag, set to 1 during the transmission, and reset to 0otherwise. The StopUnshapedTx flag is used to trigger the interruptionof the transmission of an unshaped frame when a shaped frametransmission time is due.

These operations can be implemented by a communicating entity 100 (nodeof a network, or terminal device in a peer-to-peer network, etc.),schematically shown in FIG. 4 and comprising:

-   -   one or several input ports 10 receiving the frames to process,    -   one or several output ports 14 to transmit the received frames        (and/or any other frames to be sent by the communicating entity        100 for its direct needs),    -   one or several buffer memories 13 storing at least queues of        frames before their transmission as detailed below, and    -   a computer circuit comprising for example a processor 11        cooperating with a memory 12 storing at least instructions of a        computer program for performing the method according to an        embodiment of the invention.

FIG. 5 shows an example of operation of the input ports 10 of such acommunicating entity 100.

In a first step S0, the stream to which received frames belong isidentified in the input port in order to determine whether the receivedframes are shaped frames or rather unshaped frames (step S1) and furtherin which transmit queue(s) the received frames have to be forwarded to.This identification uses information contained in the frame headerand/or payload to determine:

The destination output port (step S2), and

The destination transmit queue Q within the selected output port (stepS3).

The operation in the transmit ports 14 is now disclosed with referenceto FIGS. 6A and 6B.

In case of receiving a shaped frame (arrow Y at output of test S4) andupon storing that frame in the shaped transmit queue Qi the followingoperations are performed:

-   -   At test S5, if Stat(Qi)=0, then test S6 is performed and if        T>TTTi, then TTTi=T at step S7;    -   then TTTi value is injected in a sorting function at step S8 (as        explained in details below);    -   then the value Stat(Qi) is set to Stat(Qi)=1 (step S9).

It is added here that regarding the unshaped stream, upon storing aframe in the unshaped transmit queue Qu (arrow N from test S4) Stat(Qu)is set to not empty (=1), in step S10.

Now is disclosed the transmission of a frame belonging to a shapedstream.

The frame belonging to a shaped stream to be transmitted is selected bycomparing all TTTi (using thus the aforesaid sorting function). Theframe associated to the smallest TTTi value (as determined in step S10)is determined in step S11 as the next candidate for transmission. Forthis purpose, the sorting function maintains the smallest TTTi, with isuch that Stat(Qi)=1 (i.e. for which the queue is not empty). Thatsmallest TTTi being noted hereafter minTTTi.

If Stat(Qi) equals 0 for all streams i (i.e. all the shaped queues areempty, as determined in step S12) then minTTTi takes the null value instep S13.

If an unshaped frame (or fragment) is currently being transmitted asshown in FIG. 7A, it is checked whether it can be (re)fragmented. Tothat end:

-   -   the size of the frame (or fragment), which transmission is to be        interrupted, must be greater than MinFragSize,    -   and the size of the fragment remaining in the unshaped queue        after transmission interruption must be also greater than        MinFragSize.

If both these two conditions are not met together, the normaltransmission of unshaped frame (or fragment) continues as usual withoutfurther fragmentation, as shown in FIG. 7B.

Otherwise, as shown in FIG. 7A, when fragmentation of the unshaped frameis possible, fragments of the unshaped frame can be transmitted betweentransmissions of shaped frames.

Therefore, a shaped frame is effectively transmitted mainly when:

TTTi is due, i.e. the current time T is greater or equal TTTi;

There is no frame belonging to another shaped stream currently beingtransmitted,

There is no frame or fragment belonging to an unshaped stream currentlybeing transmitted.

Referring again to FIGS. 6A and 6B, the following operations areperformed upon each increment of time variable T by 1 time unit in stepS14. The shaped frame to be transmitted (having the lowest TTTi) asdetermined in step S11, has the index k and thus TTTk=minTTTi. In stepS15, the result of the test should be normally TTTk≠null since a frameis received at step S11. Furthermore, if T≥TTTk at step S16, then testS17 is performed so as to determine whether the transmission of anunshaped frame is currently in progress or not. If yes(UnshapedTxBusy=1), then a request for stopping that currenttransmission is set at step S18 (StopUnshapedTx=1). Otherwise (arrow Nfrom test S17), it is assessed further whether the transmission of ashaped frame is in progress or not in test S19. If not (ShapedTxBusy=0),then in step S20, the shaped frame to transmit is placed at the head ofQk.

Then, updates are performed for the next time T. In step S21, theparameter ShapedTxBusy is updated to “1” for size(frame)*8/ρ. In stepS22, the TTTk value is also updated to TTTk=TTTk+size(frame)*8/Rk, aswell as Stat(Qk) in step S23. Moreover, at test S24, if Stat(Qk) is notnull, than TTTk is injected in the sorting function again at step S25.

For the purpose of transmission and fragmentation of a frame belongingto an unshaped stream, details are now given hereafter while referringto FIG. 9. As a general rules, the transmission of a frame belonging toan unshaped stream starts if the following conditions are met:

No frame belonging to a shaped stream is currently being transmitted,

If no frame stored in one of the shaped queues has a transmission timewhich is due already (current time T greater or equal to its TTTi).

The theoretical transmission time of the next-to-be-transmitted framebelonging to a shaped stream is therefore used to check if the unshapedframe can be transmitted, or fragmented.

Finally, a first fragment of the unshaped frame is transmitted only ifboth its length and the length of the fragment remaining in Qu aregreater or equal to MinFragSize.

If not, no transmission occurs from the unshaped queue.

Indeed, as seen before, in case where a new shaped frame must betransmitted while an unshaped frame transmission is pending, thetransmission of the unshaped frame can be interrupted only if:

the length of the fragment transmitted up to the interruption is greateror equal MinFragSize,

and the length of the fragment to remain in the unshaped queue after theinterruption is greater or equal MinFragSize.

If anyone of the above conditions is not met, the transmission of theunshaped frame cannot be interrupted, as shown in FIG. 8.

Referring to FIG. 9, the following operations are performed upon eachincrement of time variable T by 1 time unit.

If UnshapedTxBusy=1 (test S26), and if StopTxUnshaped=1 (test S27), andif (sizeofQuHead≥MinFragSize and sizeofCurrFrag≥MinFragSize) in testS28, then the current frame transmission is stopped at step S29. Then,updates are performed and UnshapedTxBusy is set to null (step S30) andStopUnshapedTx is set to null (step S31).

Time T is incremented by ‘1’ (step S14) for a next run.

Otherwise, referring now to FIG. 10, if UnshapedTxBusy≠1 (arrow N fromtest S26), it is processed further to the following tests:

-   -   if Stat(Qu)≠0 (test S32), and    -   if ShapedTxBusy=0 (test S33), and    -   if min_(i) TTTi≠null (test S34),    -   if both the following conditions are met:

[(minTTTi−T)≥MinFragSize*8/ρ] and[size(frame)−(minTTTi−T)*ρ/8≥MinFragSize](test S35),

then the current frame or fragment is transmitted at the head of queueQu in step S36,

and the parameter UnshapedTxBusy is set to 1 at step S37 during theminimum value between:

-   -   (minTTTi−T) and    -   size(frame)*8/ρ (to that end, a corresponding temporisation can        be provided).

However, in test S34 (arrow N), if min_(i) TTTi=null, then in step S38,it is time to transmit the frame or fragment at the head of Qu and instep S39 to set UnshapedTxBusy to 1 during a time corresponding tosize(frame)*8/ρ.

Then, the next run is implemented when time T is incremented in stepS14.

It should be noted that residual latency can be eliminated by theprovision of a “probe” frame PF inserted in the shaped stream asexplained hereafter. Such a probe frame PF can be inserted at the headof each shaped stream queue. In fact, when typically a frame of a shapedstream is transmitted after a period of silence for example, it mayhappen the case where the transmission of an unfragmentable unshapedframe or fragment cannot be aborted (due to the size of the producedfragments for example), and it cannot be given a way to the immediatetransmission of a shaped frame instead, as shown in FIG. 7B. In order toavoid the resulting residual latency, a “probe” frame can be inserted inthe shaped stream so that the first shaped frame following this probedoes not experience any latency under such circumstances. In analternative embodiment which is shown in FIG. 3, one probe frame PF onlycan be inserted at the head of the shaped frames queue Qk (with k=2 inthe example of FIG. 3) and the next candidate for transmission in queueQk will not suffer from any latency.

More generally, the invention is targeted at packet-switched networkscapable of forwarding preemption, and can provide low latency datatransfer to time-critical applications that are transported onnon-cyclic rate-controlled streams. Typically, it can be applied inembedded control networks such as industrial or automotive networks.

1. A method implemented by computer means of a communicating entity in apacket-switched network, said communicating entity comprising at leastone port for transmitting communication signal frames, said framescomprising: a first type of frames (SF), intended to be transmitted in aplurality of streams for which a traffic shaping is defined, and asecond type of frames (UF), for which no traffic shaping is defined,each frame being able to be fragmented so as to transmit a fragment onlyof a frame of said second type, wherein the communicating entity:stores: a plurality of first queues of frames of the first type, saidfirst queues (Qi) being associated respectively to said plurality ofstreams, and at least one second queue (Qu) for frames of the secondtype, and schedules: transmissions of first type frames, and between atleast two first type frames, transmission of at least a fragment of atleast one second type frame, and wherein, for deciding to transmit afragment only of a second type frame, the communicating entitydetermines: whether the size of the fragment to be transmitted isgreater than a threshold (MinFragSize), and whether the size of aremaining fragment of said second type frame is greater than saidthreshold, and otherwise prevents from fragment transmission.
 2. Themethod of claim 1, wherein each of said first and second queues isstored in a first-in-first-out type buffer (BUFF), and wherein a queueto which a first type frame is assigned is determined according to thestream to which said first type frame belongs, depending on features ofsaid first type frame.
 3. The method of claim 2, wherein said featuresof said first type frame include a frame length and a current rate ofthe stream to which said first type frame belongs, and wherein a timetaken for the transmission of each first type frame in its queue iscalculated and respective transmission times are scheduled for saidfirst type frames according to said respective times taken for thetransmissions of the first type frames.
 4. The method of claim 2,wherein a frame belonging to a shaped stream is selected fortransmission by comparing scheduled transmission times (TTTi) of framesplaced at the head of each first queue, the frame associated to thesmallest scheduled transmission time (miniTTTi) being selected as a nextcandidate for transmission.
 5. The method of claim 4, wherein thecommunicating entity performs a time counting for determining a currenttime (T) and implements transmission of said next candidate if: thescheduled transmission time (TTTk) associated to said next candidate issmaller or equal to the current time (T), and if no other first typeframe is currently being transmitted, and if no second type frame orfragment is currently being transmitted.
 6. The method according toclaim 4, wherein the communicating entity determines whether a secondtype frame or frame fragment is being transmitted, and if yes whethersaid second type frame or frame fragment can be further fragmented forinterrupting a transmission of said second type frame or frame fragment,and allowing thereby a transmission of said next candidate.
 7. Themethod of claim 6, wherein the communicating entity implements afragmentation of said second type frame or frame fragment: if the sizeof said second type frame or frame fragment which transmission is to beinterrupted is greater than said threshold (MinFragSize), and if thesize of a frame fragment to remain in said second queue after theinterruption is greater than said threshold.
 8. The method according toclaim 1, wherein the communicating entity performs a time counting fordetermining a current time and implements a second type framefragmentation in view of fragment transmission: if no first type frameis currently being transmitted, and if no first type frame placed at afirst queue head has a scheduled transmission time (TTTi) smaller orequal to the current time (T).
 9. The method according to claim 4,wherein said communicating entity inserts a probe frame (PF) at the headof at least one of the first queues comprising said next candidate fortransmission, so as to avoid any latency of transmission of said nextcandidate.
 10. A communicating entity, comprising at least one port fortransmitting communication signal frames in a packet-switched network,and further comprising a computer circuit for performing the methodaccording to claim
 1. 11. A computer program product, comprisinginstructions for performing the method as claimed in claim 1, when runby a processor.